1. Field
This invention pertains generally to nondestructive inspection tools and methods and more particularly to such tools and methods that can be employed for inspecting difficult to access locations such as around the welds on jet pumps of boiling water reactors.
2. Related Art
A nuclear reactor produces electrical power by heating water in a reactor pressure vessel that contains a nuclear core of fissile material in order to generate steam which is used in turn to drive a steam turbine. A reactor pressure vessel of a boiling water reactor typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the reactor pressure vessel. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a boiling water reactor, hollow tubular jet pumps, positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pumps, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. The slip joint between the jet pump inlet mixer and the jet pump diffuser collar has about 0.015 inch diametrical operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump.
The inlet mixer and the diffuser, due to their large size, are formed by welding a plurality of cylindrical sections together. Specifically, the respective ends of adjacent cylindrical sections are joined with a circumferential weld. During operation of the reactor, the circumferential weld joints may experience intergranular stress corrosion cracking and irradiation assisted stress corrosion cracking in the weld heat affected zones which can diminish the structural integrity of the jet pump. Various other components and structures in the nuclear reactor have experienced similar defects because of the harsh environment. Accordingly, it is important to examine periodically these components to assess their structural integrity and determine the need for repair. Ultrasonic inspection is a known technique for detecting cracks in nuclear reactor components. Many of the areas in a nuclear reactor that need to be inspected may have limited access and therefore, are difficult to assess using an inspection tool. For example, the jet pump riser pipe and elbow welds are periodically examined for cracking. The presence of cracking can diminish the structural integrity of a jet pump riser pipe and elbow and in extreme cases adversely impact reactor coolant flow. However, the jet pump riser pipe and elbow are difficult to access. Installation access is limited to the annular space between the outside of the shroud and the inside of the reactor pressure vessel, between adjacent jet pumps. Scanning operation access is additionally restricted within the narrow space between the jet pump riser pipe and vessel, shroud, or other welded attachments such as the riser brace or restrainer brackets.
Furthermore, the inspection areas in a nuclear reactor can be highly radioactive and can pose safety risks for personnel working in these areas. Thus, inspection of these areas for the most part can require a robotic device which can be installed remotely and operated within the narrowly restricted space.
Inspecting and repairing nuclear reactors, such as boiling water reactors, typically involves manually controlled poles and ropes to manipulate servicing devices and/or positioning of these devices. During reactor shutdown, servicing of some components requires installation of inspection manipulators or devices 30 to 100 feet deep within the reactor coolant. Relatively long durations are required to install or remove manipulators and can impact the plant shutdown duration. In addition, different inspection scopes can require several different manipulators or reconfigurations requiring additional manipulator installations and removals and costs. The long durations cannot only impact plant shutdown durations, but also increase personnel radiation and contamination exposure.
Plant utilities have a desire to reduce the number of manipulator installations and removals to reduce radiological exposure as well as costs and plant outage impact. In addition, the plant utilities have a desire to reduce costs and operate as productively as possible.
Thus, it is an object of this invention to minimize the number of reconfigurations and the number of tools required to perform inspections. In addition, it is a further object of this invention to provide a means to inspect difficult to access components such as jet pump riser piping areas that have previously been inaccessible with existing tooling.